The free browser was developed by the nonprofit Mozilla Foundation in Mountain View and hundreds of volunteers relying on so-called open source software, which is available for anyone to use.

A preview version of the browser has already been downloaded 8 million times since September, raising hopes in some corners of the Internet industry that Microsoft's hold of the market may be weakening.

Chris Hofmann, director of engineering for the Mozilla Foundation, said Firefox is less vulnerable to Internet viruses and spyware than Internet Explorer and includes such features as a pop-up blocker and multiple-window browsing. However, he acknowledged that about 1 percent of the top Web sites fail to properly download with the new software.

The Mozilla Foundation was spun off last year from Netscape Communications, which played a large role in popularizing the Internet with its Navigator browser. But the company was ultimately defeated by Microsoft and subsequently acquired by America Online.

Microsoft's Internet Explorer has virtually controlled the browser market for six years. However, more recently, the software giant has shown some vulnerability.

Compared with early this year, Internet Explorer's market share was down 2 percent in October, to 93 percent, according to WebSideStory, an online consulting firm in San Diego. In addition to Firefox, other browsers include Opera and Safari, created by Apple.

Sara Radicati, chief executive for the Radicati Group, an Internet research firm in Palo Alto, was skeptical that Firefox would gain a large following. While the browser may appeal to technologically savvy users, most people will probably remain with Internet Explorer, she said.

"We think that Microsoft is very well entrenched," Radicati said. "The average person doesn't care less."